In recent years, devices are required to have smaller thickness, smaller weight, and even flexibility in association with rapid progress in electronics such as display devices (e.g., liquid crystal display devices, organic EL display devices, and electronic paper), solar cells, and touch panels. In such devices, various electronic elements such as a thin film transistor and a transparent electrode are provided on a glass plate. By replacing such a glass material with a film material, a panel itself can be reduced in thickness and in weight. However, formation of these electronic elements requires a high temperature process, and there has been no film material that can endure this high temperature process.
Moreover, in a case where these fine elements made of inorganic materials are formed on a film, there have been a risk that a difference in linear thermal expansion coefficient between the inorganic materials and the film would cause the film to warp or even break the inorganic elements after formation of the inorganic elements. In view of the circumstances, there have been demands for a film material that has the same linear thermal expansion coefficient as the inorganic materials while still having transparency and heat resistance.
Polyimides have heat resistance and high insulating performance, and have therefore been applied to electronic components. Accordingly, polyimides are often laminated with monocrystalline silicon or a metal such as copper. As such, there have been attempts to reduce the linear thermal expansion coefficient of polyimides to be as small as those of monocrystalline silicon and metals.
One of factors which greatly affect the linear thermal expansion coefficient of polyimides is chemical structures thereof. Generally, it is said that a polyimide with a more rigid and linear polymer chain has a lower linear thermal expansion coefficient. In order to reduce the linear thermal expansion coefficient of a polyimide, various structures of both tetracarboxylic dianhydride and diamine each of which is an ingredient of polyimides have been proposed.
Out of the proposed polyimides, a polyimide which contains a fluorine substituent (e.g., a polyimide obtained from 2,2′-bis(trifluoromethyl)benzidine (hereinafter referred to as TFMB)) is relatively excellent not only in heat resistance and linear thermal expansion coefficient but also in solubility in an organic solvent and transparency. Such a polyimide is reported in some literatures (for example, Patent Literature 1, Patent Literature 2, and Patent Literature 3).
For example, Patent Literature 1 and Patent Literature 2 describe thermophysical properties of a polyimide using TFMB. However, details of the other physical properties are not described in Patent Literature 1 and Patent Literature 2.
Patent Literature 3 discloses a technique regarding a soluble polyimide using TFMB. Although Patent Literature 3 mentions about a linear thermal expansion coefficient, Patent Literature 3 describes only solubility in N-methyl-2-pyrrolidone (NMP) and does not describe anything about solubility in other solvents.
As described above, although polyimides containing a fluorine atom, especially polyimides obtained from TFMB have been conventionally known, a polyimide which has solubility in various kinds of organic solvent and a low linear thermal expansion coefficient has never been disclosed.